PROJECT SUMMARY The long-term goal of this project is to obtain a better understanding of the molecular determinants for influenza A virus (IAV) infectivity and susceptibility in order to devise better strategies for prevention of IAV transmission. Our approach emanates from the existing limitations of our seasonal inactivated influenza virus (IIV) vaccine in which low vaccination coverage and vaccine effectiveness restrict the desired protection from viral acquisition, shedding, and continued transmission. This points to a critical need to devise new strategies to interrupt host-to-host transmission. Our understanding of the host factors contributing to viral contagion is very limited due to the complexities of studying natural transmission in humans and a lack of tractable animal models. Furthermore, most transmission studies in experimental models do not account for host characteristics that could affect IAV transmissibility and susceptibility. Hence, we recently developed an infant mouse model of intra-litter IAV spread that enables elaborate studies of the biology of influenza virus transmission. From this model, we established that high titers of virus shed by the infected donor correlates with the efficiency of transmission, which is dependent on various host factors (such as age and immunity) and viral factors (such as hemagglutinin and neuraminidase). Furthermore, manipulations of the upper respiratory tract (URT) environment (via Streptococcus pneumoniae colonization) revealed that bacterial sialidase expression limited the IAV acquisition by recipients. Together, our preliminary data suggested that the infectivity of IAV relies on the amount of shed virus from the donor to overcome a transmission bottleneck, whereas the susceptibility to IAV relies on the availability of sialic acid in the URT of recipients which may vary, based on the composition of the URT flora. The molecular determinants of virus shedding and sialic acid availability in the URT have not been the focus of previous investigation, vaccine, or therapeutic design. Therefore, in Aim #1, we will evaluate the IAV transmission bottleneck by defining viral and host factors influencing the infectivity (via shedding) of the donor, whereas in Aim #2, we will evaluate the role of sialic acid in the susceptibility to IAV infection through transmission. At the conclusion of the project, we will understand the roles of the host and virus in the infectivity and susceptibility to IAV. We hope to determine whether virus shedding and/or sialic acid availability can be targeted in future studies, as a novel approach to inhibit IAV contagion.